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Javascript shift()

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  Javascript shift() JT <jonas.thornvall@gmail.com> - 2013-06-14 00:15 -0700
    Re: Javascript shift() Denis McMahon <denismfmcmahon@gmail.com> - 2013-06-14 11:29 +0000
      Re: Javascript shift() Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2013-06-14 15:01 +0200
    Re: Javascript shift() Stefan Weiss <krewecherl@gmail.com> - 2013-06-14 15:03 +0200

#20284 — Javascript shift()

Can i somehow tell which javascript elements to shift?
Is something like following possible
element[i].shift()

Many times one want to change items not necessarily at element[0], and
sometimes you want to swap them with another index.

I think the shift function should take one or two parameters or a new
function should be constructed.

element [x][y].swap();

It is not hard to swap elements but it make more sense then the
noneflexible shift() function.

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#20286

On Fri, 14 Jun 2013 00:15:37 -0700, JT wrote:

> Can i somehow tell which javascript elements to shift?
> Is something like following possible element[i].shift()
> 
> Many times one want to change items not necessarily at element[0], and
> sometimes you want to swap them with another index.
> 
> I think the shift function should take one or two parameters or a new
> function should be constructed.
> 
> element [x][y].swap();
> 
> It is not hard to swap elements but it make more sense then the
> noneflexible shift() function.

If you want to swap elements x and y in an array:

element [x][y].swap();

makes no sense.

Your function would have to be something like:

array.swap_elements(x,y);

But why would you ever need to do this except in the context of sorting? 
An array is either ordered or unordered - if it's unordered it doesn't 
matter what order the elements are in, and if you want it ordered 
according to some specific algorithm, just write a custom sort function 
and sort with it:

array.sort(sortfunction);

where:

sortfunction( a, b ) {}

returns <0 if a comes first, >0 if b comes first, 0 if they're equal.

-- 
Denis McMahon, denismfmcmahon@gmail.com

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#20287

Denis McMahon wrote:

> On Fri, 14 Jun 2013 00:15:37 -0700, JT wrote:
>> Can i somehow tell which javascript elements to shift?
>> Is something like following possible element[i].shift()
>> 
>> Many times one want to change items not necessarily at element[0], and
>> sometimes you want to swap them with another index.
>> 
>> I think the shift function should take one or two parameters or a new
>> function should be constructed.
>> 
>> element [x][y].swap();
>> 
>> It is not hard to swap elements but it make more sense then the
>> noneflexible shift() function.
> 
> If you want to swap elements x and y in an array:
> 
> element [x][y].swap();
> 
> makes no sense.

ACK.

> Your function would have to be something like:
> 
> array.swap_elements(x,y);
> 
> But why would you ever need to do this except in the context of sorting?

One can never know.

> An array is either ordered or unordered - if it's unordered it doesn't
> matter what order the elements are in,

Your logic is flawed – again.

> and if you want it ordered according to some specific algorithm, just
> write a custom sort function and sort with it:
> 
> array.sort(sortfunction);
> 
> where:
> 
> sortfunction( a, b ) {}
> 
> returns <0 if a comes first, >0 if b comes first, 0 if they're equal.

As that is a syntax error, it does not return anything.  Functions are 
declared with

  function identifier (parameter1, parameter2/*, …*/)
  {
    // …
  }

and called with

  identifier(argument1, argument2/* , … */);

[The number of arguments does not need to match the number of formal 
parameters.  If an argument is passed for which there is no parameter, its 
value is available via a numeric zero-based property of the “arguments” 
object in the function context.  If no argument is passed for a formal 
parameter, it is initialized with the “undefined” value.  Also, the value of 
the “arguments.length” property yields the number of arguments that have 
been passed to a function.]

It is not always necessary to declare a function as there are function 
expressions:

  array.sort(function (a, b) {
   // …
  });

or

  var sortNumerically = function (a, b) {
    return a - b;
  };

  array.sort(sortNumerically);

A function declaration and the latter approach has the advantage that an 
unchanging function needs only be created once.  The advantage of a function 
expression over a function declaration primarily is that its evaluation 
result (a Function instance reference) can be assigned as value to any 
property, allowing for namespacing.

[It is strongly recommended to define and use this trivial sort function, as 
the default is implementation-dependent and may sort the elements as string 
values instead.]

See also: <http://PointedEars.de/es-matrix#features>

-- 
PointedEars

Twitter: @PointedEars2
Please do not Cc: me. / Bitte keine Kopien per E-Mail.

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#20288

On 2013-06-14 09:15, JT wrote:
> Can i somehow tell which javascript elements to shift?
> Is something like following possible
> element[i].shift()
> 
> Many times one want to change items not necessarily at element[0], and
> sometimes you want to swap them with another index.
> 
> I think the shift function should take one or two parameters or a new
> function should be constructed.

You're probably looking for the splice() method. To insert an element at
index 2:

  var a = [2, 3, 5, 6];
  a.splice(2, 0, 4);
  // a is now [2, 3, 4, 5, 6]

shift()/unshift() and push()/pop() always work on the ends of an array;
you could think of them as specialized cases of splice().

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/splice

> element [x][y].swap();
> 
> It is not hard to swap elements but it make more sense then the
> noneflexible shift() function.

Why would you need shift() for element swapping? Something like this
should be sufficient (untested):

  Array.prototype.swap = function (a, b) {
      var tmp = this[a];
      this[a] = this[b];
      this[b] = tmp;
  };

  var a = [2, 3, 5, 4, 6];
  a.swap(2, 3);
  // a is now [2, 3, 4, 5, 6];

(BTW: better use Object.defineProperty - where supported - instead of
directly augmenting a native prototype.)


- stefan

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